1. Field of the Invention
The present invention relates to a high-voltage generation apparatus, and more particularly, to a high-voltage generation apparatus capable of raising the high voltage to a target voltage at high speed.
2. Description of the Related Art
In recent years, in an electrophotographic color image forming apparatus, a configuration including a photosensitive drum serving as an image bearing member for forming an image to increase a print speed for each color (also referred to as a tandem type color image forming apparatus) has been mainstream. In the tandem type color image forming apparatus, a color misregistration detection pattern image and a density detection pattern image are formed on an intermediate transfer belt, for example, and an optical sensor detects light reflected from a patch image, to perform color misregistration correction and density correction (the corrections are also referred to as calibrations) based on a detection result. The calibrations are mainly executed at timing, for example, at the time of replacement of a cartridge including a photosensitive drum, at the time of power application, and after a lapse of a predetermined time. The color misregistration detection pattern image and the density detection pattern image are together merely referred to as a patch image.
In the color image forming apparatus, when image formation is continuously performed (hereinafter also referred to as “continuous printing is performed”) on a plurality of recording materials, a temperature in the apparatus rises. This rise in temperature causes the recording materials to be deformed or distorted due to effects of rises in temperature of a scanning optical unit for forming a latent image on a photosensitive drum and components in the unit so that a color misregistration amount of the image may be increased. A rise in temperature of the photosensitive drum by continuous image formation causes an image formation condition to change so that a density of the image may vary. In order to reduce a color misregistration and a density variation due to the rises in temperature, the calibrations may be executed when the rise in temperature is monitored to enter a predetermined condition even in the continuous image formation. However, an image forming operation is temporarily interrupted due to the calibrations during the continuous printing, resulting in significantly reduced productivity.
A technique for sequentially executing calibrations to correct a color misregistration and a density while performing continuous printing without reducing productivity during the continuous printing has been proposed. For example, there is a method for forming and detecting a patch image a plurality of times for each color in a non-image formation area between a trailing edge of an image and a leading edge of the subsequent image (also referred to as between images or between sheets). Thus, density control can be performed without temporarily interrupting image formation. The patch image formed in the non-image formation area between sheets is recovered by a similar cleaning mechanism to that during recovery of toner remaining on a transfer belt, e.g., a cleaning blade that abuts on the transfer belt. An apparatus using an electrostatic transfer belt as the transfer belt can clean the patch image without any difficulty. However, in an intermediate transfer belt type apparatus, a nip portion between a secondary transfer roller and an intermediate transfer belt is arranged upstream in a toner image conveyance direction of the cleaning blade. More specifically, the patch image adheres to the secondary transfer roller, and adheres to the back of a recording material that then passes through the nip portion so that the back of the recording material may be smudged. In the intermediate transfer belt type apparatus, a method for instantaneously switching a secondary transfer bias to a negative bias according to timing at which the non-image formation area passes through the nip portion to prevent the back of the recording material from being smudged is required. A circuit for outputting the secondary transfer bias is a circuit giving priority to stability during steady output. Times required to raise and lower a high voltage are approximately 50 ms to 100 ms.
In order to increase the speed of the color image forming apparatus, to form a patch image between sheets without reducing productivity, an electrical technique for applying a bias having a polarity opposite to that of a bias in an image formation area in a short time is required. More specifically, the above-mentioned times required for rise and fall are required to be further shortened. Japanese Patent Application Laid-Open No. 2008-58510 discusses a technique for shortening a time required for fall in a configuration in which high-voltage power sources having a positive polarity and a negative polarity are provided, and are respectively turned off and turned on according to timing at which a patch image passes through a transfer nip portion. In Japanese Patent Application Laid-Open No. 2008-58510, a capacitor for rectifying and smoothing an alternating-current (AC) voltage output from a transformer for a positive bias and an electric charge charged in a capacitance of a load unit are drawn into a power source for a negative bias so that a voltage level is rapidly reduced. As a result, a time required to fall for switching an output of a high-voltage bias from a positive polarity to a negative polarity is shortened to approximately 10 ms to 20 ms (see FIG. 4 in Japanese Patent Application laid-Open No. 2008-58510). Even if a time between sheets becomes short, therefore, an output of a transfer bias can be switched to a negative polarity within a time elapsed since a trailing edge of a recording material passes through the transfer nip portion until the recording material reaches a leading edge of the patch image.
As an example in which a high voltage is raised at high speed, Japanese Patent Application Laid-Open No. 9-93920 discusses a technique for comparing a detected voltage of a voltage detection circuit with a second reference voltage slightly lower than a reference voltage, to slowly control a rate of charging a capacitor serving as a load when the detected voltage of the voltage detection circuit exceeds the second reference voltage. In Japanese Patent Application Laid-Open No. 9-93920, a fast charging area, a slow charging area, and a maintenance charging area are provided in this order from the time of startup. After the start of the startup, a pulse width modulation (PWM) signal is rapidly raised by increasing its value (a time width of a high-level pulse out of high-level and low-level pulses of the PWM signal; hereinafter referred to as an on-duty width) so that an ON time of the PWM signal becomes the maximum time width. When an output voltage, which reaches the second reference voltage (exemplified as approximately 90%), is detected, switching from the fast charging area to the slow charging area is performed. An integration circuit is provided on the input side of a circuit for generating a pulse of the PWM signal. The capacitor is rapidly charged in an early stage of the time of the startup, and is then slightly charged and discharged in the slow charging area and the maintenance charging area, to reduce an overshoot or an undershoot.
Even if a time required to lower a high voltage is shortened to approximately 10 ms to 20 ms when the high voltage is switched from a positive polarity to a negative polarity, as discussed in Japanese Patent Application Laid-Open No. 2008-58510, when an image formation speed is further increased so that a time between sheets is further shortened, a time required to form a patch image becomes difficult to ensure. FIGS. 1A to 1C illustrate a relationship among the time between sheets, a time required to switch the high voltage, and an operation for forming the patch image. A time during which the patch image can be formed in a non-image formation area between sheets is a time obtained by subtracting [a time required to raise a secondary transfer bias plus a time required to lower the secondary transfer bias] from a non-image formation time. When the time between sheets is further shortened, a ratio of the times required to raise and lower the secondary transfer bias to the non-image formation time is increased. Therefore, there is almost no time required to form the patch image, as illustrated in FIG. 1A. In order to ensure a time required to form the patch image, the time between sheets may be lengthened, as illustrated in FIG. 1B. However, a time required to detect both a color misregistration amount and a density between sheets, i.e., [a time required to form the patch image plus a time required to lower the high voltage plus a time required to raise the high voltage] is ensured so that a time between sheets is significantly lengthened, resulting in reduced productivity. When the time between sheets is further shortened, therefore, a startup capability of the high-voltage generation apparatus (hereinafter described as a capability representing the magnitude of a voltage for raising a potential at a load output unit per unit time) is further required to be rapidly improved to switch a polarity of a high-voltage output unit at high speed, as illustrated in FIG. 1C.
An issue occurring in the high-voltage generation apparatus discussed in Japanese Patent Application Laid-Open No. 9-93920 as another example will be described below with reference to FIGS. 2A and 2B. A waveform up illustrated in FIG. 2A is an example of a waveform of a positive-polarity bias raised toward a target voltage of +2 kV, for example, using the high-voltage generation apparatus discussed in Japanese Patent Application Laid-Open No. 9-93920. On the other hand, waveforms βp1 and βp2 are examples of a waveform generated when a load unit is charged with a negative potential having an opposite polarity before the high-voltage generation apparatus is started up. A time required for the startup is lengthened by an amount corresponding to a time required to discharge an electric charge charged at the negative potential. More specifically, a time required to switch a polarity of a bias at the time of transition from a non-image formation area to an image formation area, described with reference to FIGS. 1A to 1C, is lengthened. FIG. 2B illustrates a case where a high-voltage generation circuit having a negative polarity is used. A waveform αn is an example of a waveform of a negative-polarity bias raised toward a target voltage of −2 kV. On the other hand, waveforms βn1 and βn2 are examples of a waveform generated when a load unit is charged with a positive potential having an opposite polarity before the high-voltage generation apparatus is started up. A time required for the startup (a time elapsed until a potential is lowered so that its polarity is changed to a negative polarity) is lengthened by an amount corresponding to a time required to discharge an electric charge charged at the positive potential.